Carbon Fiber AFO’s: Not all Carbon foot prints

are the same

Travis Carlson, LPO/Partner

Relevant to the content of this educational activity, Mr. Carlson indicated he has no financial

relationships with commercial interest companies to disclose.

Objectives

• Discuss the “where and why” of carbon fiber.

• Take a brief look at the literature.

• Discuss the significant increase in utilization.

• Un-pack the different functional mechanics of three

main manufactures of Carbon AFO lines.

• Free for all Q and A.

Definitions

• Kinematics – Study of the

relationships between

displacement, velocity, and

acceleration.

• Kinetics – Study of moving

bodies, including FORCES

producing motion.

Planes of Motion

Temporal Parameters

• Step Length – The distance from one event (usually initial contact)

of one foot to the subsequent occurrence of the other foot.

• Example: Heel strike of one foot to heel strike of the other foot.

• Stride Length – The distance from one event (usually initial contact)

of one foot to the subsequent occurrence of the same foot. • Example: Heel strike of one foot to heel strike of the same foot.

Temporal Parameters

Velocity:

• The average horizontal speed of the body along the line of

progression.

• Velocity is measured over several strides

• m/sec. or mi/hr.

• Freely selected speed for walkers = 1.3 → 1.4 meters/second

Cadence:

• The number of steps per unit of time.

• Steps/minute

• Freely selected speed for walkers = 1.3 → 1.4 meters/second

Average Velocity

• Stride length ᵡ ½ Cadence

• Step length ᵡ Cadence (assuming symmetry)

Speed…Increased Speed…

• Both cadence & step length increase

• Double support decreases with

increase speed!

Out walking the Grim Reaper…

Muscular

Contractions

Concentric

Length of muscle shortens

Muscle force is greater than the resistance

Isometric

Active constant length contraction

Muscle force is equal to the resistance

Eccentric

Active lengthing contraction

Muscle force is less than the resistance

Gait

Cycle

• Gait Cycle - The period of time from one event (usually initial

contact) of one foot to the subsequent occurrence of the same foot.

• Example: Heel strike of one foot to heel strike of the same foot.

• Stance Phase – The period in time when the foot is in contact with

the ground.

• In normal gait = 60%

• Swing Phase – The period in time when the foot is NOT in contact

with the ground.

• In normal gait = 40%

• Double Limb Stance – The period in time when BOTH feet are in

contact with the ground.

• This occurs 2x’s within a gait cycle: At the beginning and end of stance phase.

Gait Terminology

• Line of Progression – A line that is parallel to the

direction that the patient is walking.

• Toe Out (Foot Progression Angle) – A line that is

parallel to the direction that the patient is walking.

• Approx.= 5°-7° degrees

• Females prefer less than males.

• Base of Support– Area bounded by the perimeter

of the feet.

• Statically = approx. shoulder width

• Dynamically = 2”-4”

C O M (Center of Mass) & Weight Line

• Center of Mass – Average location of the mass

where the total mass can be assumed concentrated.

• Standing posture = anterior to S2

• 56% of body height

• Weight Line – Also know as the line of

gravity.

• Plumb bob

GRF – Ground Reaction Forces• Ground Reaction Force – The reaction force of

the body (usually the foot) as it interacts with the

ground.

• Newton’s 3rd Law –

• 3-dimensional vector that varies with time.

• Vertical force required to counteract the pull

of gravity.

• Shear forces required to maintain body

balance and to change speed or direction of

walking.

GRF – 3 dimensional factorsMedio-Lateral Forces:

• Small, tends to be ignored.

• For most of stance phase, it tends to accelerate the

body’s COM towards the contralateral (medial) side of

the body.

Fore-aft Forces:

• Initial: “Digging in of Heel”

• Middle: Body pushing forward into the ground.

• End: Body pushing backward into the ground.

Vertical Forces:

• Body COM is accelerating upward when the vertical

GRF is greater than the body weight.

• Body COM is accelerating downward when the vertical

GRF is less than the body weight.

Moments

• Moment of Force:

• External Moment – Rotation of segments created by external loads

(forces outside the body).

• Examples: gravity, inertia, and GRF

• What you see!

• Internal Moment – Bodies “rotational”, muscular and/or soft tissue

response to these moments created by external loads.

• Body’s response

• Data from most Motion Analysis Labs

Moments continued…

Disclaimer:

• Moments we talk about in this lecture represent

NET joint moments

• Muscle co-contractions are not taken into

account.

• Zero moments about a joint doesn’t necessarily

mean there are no forces acting across the joint –

two opposing forces could create moments of

equal magnitude that cancel each other out.

Joint Power…

Power Absorption:

• If a joint moment and angular velocity are moving in OPPOSITE

directions = usually Eccentric Contraction

Power Generation:

• If a joint moment and angular velocity are moving in the SAME

direction = usually Concentric Contraction

Moment Example• During Loading Response of gait, what effect does the

GRF vector have on the knee?

• Does the GRF Flex or Extend the Knee?

• Is this an internal or external moment?

Flexes the Knee!

External Knee Flexion Moment!

• During Loading Response of gait,

what are the muscles of the knee

trying to do?• Are the muscles flexing or extending

the knee?

• Is this an internal or external moment?

Extending the knee – controlling the rate

of knee flexion; eccentric contraction.

Internal Knee Extensor Moment!

Carbon fiber production

The manufacturing process is

expensive and nasty.

Precursors of plyacrylonitrile and

or petroleum pitch are the

foundations.

Add a nasty mix of chemicals, gas

and heat within a vacuum and

there you have it.

There are only 12 main producers

of carbon fiber in the world.

Carbon Fiber is heterogeneous as

opposed to steel which is

homogenous.

Material Science of

Carbon Fiber

The strongest carbon fiber is about five

times stronger than steel while being a

fraction of the mass.

Carbon fiber is extremely resilient and

therefore energy efficient.

Carbon fiber can be manipulated over

and infinite amount of shapes and

designs.

Variability of epoxy resins can alter the

raw stress/strain data.

What does the literature say?

• 2004 Swedish study found the use of carbon fiber AFO’s

increased walking speed 20% and decreased energy cost by

12%. (1)

• 2007 German study found carbon afo’s significantly increased

energy return at third rocker and kinematic analysis of the ankle

and knee showed more physiologic gait. (2)

• 2010 Netherland study explored the relationship between

energy return, stiffness and timing. Lowest energy cost

determined by stiffness associated with highest ankle push off

velocity just before contralateral foot strike. (3)

Utilization

• Prospective payment/ DRG

• Two day rule

• LCD documentation requirements for custom

fabrication vs OTS carbon

• Lighter than custom poly pro

• Easier to don/doff, wider selection of shoe

possibilities

• Dynamic

• Patient compliance

THREE BIG HITTERS

Main design options

• Anterior approach

– Easy donning

– Smooth transition at heel strike

– Strong influence to the knee mid to late

stance

– Great for partial foot amputees

– Not indicated if hyperextension of the knee is

present

Main Design Options

• Posterior approach

– Strong knee flexion moment at heel strike

– Less visually obvious

– Most benefits during swing phase

– Not indicated if quads are involved

Main design options

• Medial strut

– Slight medial bias during swing

– Indicated for neutral and varus foot structures

– Heavy pronators with struggle with comfort

Main Design Options

• Lateral strut

– Slight external rotation during swing

– Indicated for neutral or valgus foot structures

– Strong supinators will struggle with comfort

Ottobock Walk On

Medial strut/anterior entry

Flat stress/strain curve

Soft heal strike

Smooth stance transitions

Poor knee flexion/extension

control

Ottobock Walk On Flex

Similar in design as the Walk On

Similar stress/strain fingerprint as

Walk ON

Spiral shape produces external

rotation moment

Walk ON Reaction

Posterior entry

Medial strut

Does have lateral T-strap option for

control of the ankle

Shorter than other anterior

approach options

Allard Toe OFF

Designed for late stance

instabilities

Soft heel strike

User must have good posterior

knee control

Thusane SpryStep

Very rigid

Lateral strut

Anterior entry

Strong knee flexion moment at

initial contact

User must have good anterior

control of the knee

SpryStep Max

Posterior entry

Lateral strut

Most “action” at late

stance

SpryStep Plus

True ground reaction design

Strut placement and overall weight

may limit use

Limited experience at this time

Overall Goals of Gait

1. Propulsion

2. Stance Stability

3. Shock Absorption

4. Energy Conservation

Initial Contact Goals

Goals:

• Transfer weight to Stance

Phase Limb

• Shock Absorption

Loading Response Goals

Goals:

• Shock Absorption

• Completely Transfer Weight

• Begin Single Limb Stance

Phase Limb

Midstance Goals

Goals:

• Maintain Momentum

• Highest Position (greatest

potential energy)

• Slowest Velocity in Gait Cycle

Terminal Stance GoalsGoals:

• Allow mass to progress over the

foot for an adequate step length.

• Opposite foot contacts the

ground.

• Downward movement of the

body (velocity increases).

Pre-Swing Goals

Goals:

• Preparation of limb for Swing

Phase.

• Aid in transferring load to

opposite limb.

Initial Swing Goals

Goals:

• Continued Plantar Flexion (via

inertia) followed by Rapid Ankle

Dorsiflexion.

• Continued Knee Flexion.

• Hip Flexion used for Swing Phase

Clearance.

• Preparation for Swing Phase

Clearance.

Mid-Swing Goals

Goals:

• Continue with Dorsiflexion for

Swing Phase Clearance.

• Maintenance of Knee Flexion for

Swing Phase Clearance.

• Continue with Hip Flexion for

Swing Phase Clearance.

Terminal Swing Goals

Goals:

• Return to neutral position.

• Preparation for initial contact

5 Pre-requisites of GaitPerry & Gage

1.Stability in Stance

2.Clearance in Swing

3.Pre-Position of the foot in Terminal Swing

4.Adequate Step Length

5.Energy ConservationReference:

References

• 1. Danielsson A, Sunnerhagen K. Energy expenditure

in stroke subjects walking with a carbon composite ankle

foot orthosis. J Rehabil Med 2004;36(4);165-168.

• 2. Alimusaj M, Knie I, Wolf S, et al. Functional impact of

carbon fiber springs in ankle foot orthosis. Orthopade

2007;36(8):752-756.

• 3. Bregman DJ, van der Krogt MM, de Groot V, et al.

The effect of ankle foot orthosis stiffness in the energy

cost of walking: a simulation study. Poster presented at

13th ISPO world conference, Leipzig Germany, May

2010.

Thank You!